If you've got a multi-monitor setup, DisplayFusion offers more monitor customizations and can also load photos similarly from popular online sources. There's both free and Pro versions (you'll need the $25 Pro license for random image rotating).

If you're a fan of Microsoft's Bing homepage images, the Bing Desktop program will sync your desktop background with the daily Bing.com wallpaper. It also adds a Bing search bar and news headlines to your desktop.

There are many others, including Wallpaper Juggler, which downloads images from interfaceLIFT and WallpaperStock, but the ones above are the most feature-rich.

If you're browsing Flickr and find a photo you'd like as a wallpaper, this IFTTT recipe makes it easy to do that. Any time you favorite a Flickr photo, it will send it to where you want to put it in Dropbox. Make that Dropbox folder your wallpaper folder, and that's it. You can create (or find) similar recipes for Facebook and Picasa on IFTTT.

Using the solutions above, you should never have to worry about running out of wallpapers or being bored by your desktop. Enjoy!